Shock waves are propagating pressure pulses in elastic media, such as air, water and human/animal tissue. Acoustic shock waves have been used for various medical purposes as a noninvasive and non-surgical treatment. It has been proven to be effective to treat a variety of medical conditions in various clinical practices and research reports. For example, in urology, high-intensity focused shock waves are used for breaking kidney/bladder/urethra stones into small fragments on the order of several millimeters in diameter (i.e., lithotripsy), so that the small pieces can be transported out of the patient's body through the urethra. In orthopedics, shock waves are used for pain and inflammation relief/curing in joints and healing of bones. It is also shown that shock wave therapy is effective for healing wound, revascularization, and Peyronie's disease.
Acoustic shock wave generation is often based on three different mechanisms: electrohydraulic, electromagnetic, and piezoelectric. In the electrohydraulic method, a pulse electric discharge between two closely positioned electrodes inside water induces a sudden vaporization of small amount of water nearby. This rapid increase of volume caused by the vaporization creates a pressure pulse in the water, thus generates radial propagating shock waves. In the electromagnetic method, an electric current pulse in a conductor coil results in a pulsed electromagnetic field, which in turn repels a conductive film having certain elastic properties and positioned closely to the coil, thereby generating a momentary (e.g., pulsed) displacement in the conductive film. The momentary displacements in turn generate shock waves with wave fronts parallel to the metal film surface. Alternatively, in the piezoelectric shock wave generation method, electrical voltage pulses are applied to an array of piezoelectric ceramic tiles. The voltage pulses induce volume expansions and contractions of the ceramics with each, thereby generating shock waves with wave fronts parallel to the ceramic surfaces.
For all the three different methods, the original designs of the shock wave generators are targeting one or more focal points or a focal volume. This is realized utilizing either reflection of a radial (electrohydraulic) wave by an ellipsoidal surface to redirect the waves, or generation directly from a partial spherical surface generator (electromagnetic or piezoelectric). There is also prior design for generating plane wave or nearly plane wave by reflecting using a parabolic surface. All these prior arts share a key feature that the shock wave transducers have a window through which shock waves are emitted, and this window is opened towards a specific direction regardless of convergent or divergent shock waves. Considering the energy flow of the shock waves for the prior arts, the shock wave energy is exiting the window and propagating away from the window towards the target. For example, FIG. 1A shows a focusing device with a point source (usually realized using electrohydraulic method) located in one focal point of an ellipsoid. The radial generated shock waves are reflected by the ellipsoidal surface and become focused on the other focal point of the ellipsoid outside an exit window of the generator. FIG. 1B shows a device for generating planar shock wave by reflecting the shock waves generated by a point source using a parabolic curved surface. By modifying the shape of the reflection surface or the shape of the surface generator, the shock wave emission can be changed from convergent to divergent. These are all described in prior arts. All the prior arts have a share feature, which is an exit window and a certain direction of transmission regardless of convergent, divergent, or planar.